1. Field of the Invention
The present invention relates in general to a semiconductor device, such as an MOS transistor, in which there is compensation for the drop in the threshold voltage (Vth) due to the short-channel effects, and to a process for fabrication of such a semiconductor device.
2. Description of the Related Art
For a given nominal channel length (L) of a transistor, the threshold voltage (Vth) drops suddenly, in particular for short-channel transistors (i.e., those having a channel length of less than 0.25 μm and typically a channel length, L, of about 0.18 μm).
The threshold voltage of a semiconductor device such as an MOS transistor, in particular a short-channel device, is a critical parameter of the device. This is because the leakage current of the device (for example, of the transistor) depends strongly on the threshold voltage. Taking into consideration current supply voltages and those envisaged in the future (from 0.9 to 1.8 volts) for such devices and the permitted leakage currents (Ioff of approximately 1 nA/μm), the threshold voltage Vth must have values of approximately 0.2 to 0.25 volts.
The sudden voltage drop (or roll-off) in the zones of the channel region of the semiconductor device results in dispersion of the electrical characteristics of the device and makes it difficult to obtain the desired threshold voltages.
To remedy this threshold voltage roll-off in semiconductor devices such as MOS transistors, it has been proposed, as described in the article “Self-Aligned Control of Threshold Voltages in Sub-0.02-μm MOSFETs” by Hajima Kurata and Toshihiro Sugii, IEEE Transactions on Electron Devices, Vol. 45, No. Oct. 10, 1998, to form, in the channel region, pockets adjacent to the source and drain region junctions that have a conductivity of the same type as the substrate; but in which, the dopant concentration is greater than that of the substrate.
Although this solution reduces the threshold voltage roll-off gradient in the channel region, the short-channel effects lead to a more rapid roll-off of the threshold voltage, Vth, than the increase in the threshold voltage that can be obtained by incorporating the compensation pockets of the prior art.
Consequently, although these compensation pockets allow partial local compensation for the roll-off of the threshold voltage, Vth, it is not possible to obtain complete compensation for the roll-off over the entire channel region range desired.
Therefore a semiconductor device, such as an MOS transistor, that remedies the drawbacks of the devices of the prior art may be desired.
More particularly, a semiconductor device, such as an MOS transistor, whose voltage threshold roll-off due to the short-channel effects is almost fully compensated for may be desired. This makes it possible to achieve channel lengths which are arbitrarily small but non-zero.
Also a semiconductor device, such as an MOS transistor, may have a constant threshold voltage, Vth, when the channel length, L, decreases down to very small effective channel lengths, for example, 0.025 μm or less.
A process for fabricating a semiconductor device may apply to devices having channels of arbitrarily small length, these being, moreover, technologically realizable.